How to Make White Transparent in a Shader
There are many ways to do it, here it's one of them:



In your color ramps you need to replace the white color with a transparent one (just set the Alpha value to 0). That way you can have a white background and add the colors on top by using the Alpha output of the color ramps.

An alternative way could be to use the black color instead of the alpha and choose Screen or Add as blend modes